1. Field of the Invention
This invention relates to propulsion systems. Specifically, the present invention relates to an afterburner for use on a jet engine.
2. Description of the Related Art
Tremendous advances have been made within the first century of aviation. One of the most important advances is the jet engine. Currently, jet engines fall into two broad categories, turbojet and turbofan systems. FIG. 1 illustrates a simplified schematic drawing of a conventional turbojet engine 10. The turbojet engine includes a low pressure compressor 12, a high pressure compressor 14, a combustor 16, a high pressure turbine 18, and a low pressure turbine 20. In the turbojet engine, air comes through an inlet 22 and passes through each component of the engine. The compressor raises the pressure of the inlet air. The pressure ratio varies for different engines, but may approach 30 to 1. The high pressure air enters the combustor where fuel is injected. The fuel-air mixture is ignited and the resulting hot gases pass though the turbines, which in turn, drive the compressors. The air is exhausted through an exhaust 24 and provides the thrust that propels the aircraft. FIG. 1 illustrates a typical twin-spool engine. In addition, the low pressure compressor is driven by the low pressure turbine and the high pressure compressor is driven by the high pressure turbine. These two units rotate at different speeds in order to maintain high efficiency in all stages of compression.
Although the turbojet engine 10 provides significant thrust to propel an aircraft, many aircraft, such as military jet fighters, require large, short time increases in thrust. For example, during takeoff, climb, acceleration, and air combat maneuvers, increased thrust is critical to the utility and safe performance of the aircraft. Quite often an afterburner component is utilized. The afterburner injects additional fuel directly into the engine exhaust and burns in the tail pipe. FIG. 2 illustrates a simplified schematic of a turbojet engine 10 having an afterburner component 30. The afterburner component includes an afterburner duct 32 housing a plurality of axially aligned fuel spray bars 34, a plurality of axially aligned flame holders 36, and an adjustable nozzle 38. Fuel is injected into the exhaust of the rotating part of the engine by the fuel spray bars, and the flame holders stabilizes the flame and prevents the flame from being blown out the exhaust section of the turbojet engine. To obtain maximum thrust from the engine in afterburning and non-afterburning operation, the adjustable nozzle is utilized. The nozzle varies in size and shape to maximize the performance at all flight and engine-operating conditions. Thrust increases of approximately 50 to 80 percent are achievable by use of afterburners on modern jet engines.
FIG. 3 illustrates a simplified schematic drawing of a conventional turbofan engine 40. The turbofan engine contains many of the same components as the turbojet engine 10, such as the low pressure compressor 12, the high pressure compressor 14, the combustor 16, the high pressure turbine 18, the low pressure turbine 20, an inlet 22 and an exhaust 24. However, the turbofan engine utilizes hot jet exhaust extracted by a turbine to drive a fan 42. A portion of the inlet air that enters the fan is bypassed around the engine. The fan, thus, functions in a somewhat similar fashion to a propeller being driven by the turbo-machinery. However, unlike the propeller, a single fan stage may contain from 20 to 50 blades and is typically surrounded by a shroud and actually operates much like a single-stage compressor rather than a propeller. For example, the pressure ratio across a single fan stage is typically in the range of 1.4 to 1.6 while the pressure ratio across the propeller discs of the Lockheed Super Constellation in cruising flight is somewhat less than 1.02.
The bypass ratio of a turbofan engine is defined as the ratio of the mass of air that passes through the fan, but not the gas generator, to that which does pass through the gas generator. The larger the bypass ratio, the greater the amount of energy extracted from the hot exhaust of the gas generator, by as much as 75 percent of the total thrust of a turbofan engine may be attributed to the fan.
Most modern civil and military aircraft are powered by some form of turbofan engines because such engines consume far less fuel to produce a given amount of useful power than do comparable turbojet engines. The higher efficiency of the turbofan engine can be explained with the use of Newton's second law of motion. From this law, it may be deduced that a given level of thrust can be produced at a given flight velocity, either by the addition of a small increment of velocity to a large mass flow of air or by the addition of a large increment of velocity to a small mass flow of air. The required energy addition (i.e., fuel), however, is less for the first than for the second case. The improved efficiency of the turbofan engine as compared with the turbojet is, therefore, directly related to the larger air-flow capacity of the fan engine at a given thrust level.
In a similar manner as discussed in FIG. 2 for a turbojet engine, an afterburner component 50 may be utilized in the turbofan engine. FIG. 4 is a simplified schematic drawing of the afterburner component for use on the turbofan engine 40. The afterburner component 50 includes an afterburner duct 52 housing an axially aligned plurality of fuel spray bars 54, a plurality of axially aligned flame holders 56 and an adjustable nozzle 58. Afterburning in a turbofan engine typically take place in a mixture of the primary exhaust air and the fan bypass air. Alternative, a duct burning turbofan may be utilized where the fuel spray bars and the flame holders are located in the fan duct and all the afterburning takes place in the bypass air.
For both turbojet and turbofan engines, afterburner operation is feasible because a jet engine operates at a “lean” fuel-to-air ratio to limit temperatures in the hot, rotating parts of the engine to values consistent with high temperature limitations of the materials with which these parts are constructed. Thus, the turbine exhaust provides the excess oxygen necessary for afterburner operation. The afterburner component on a jet engine provides a light and mechanically simple means for achieving a large boost in thrust. However, use of afterburner suffers from significant increase in fuel flow. In addition, drag is increased by the use of the modified jet engine. A jet engine is needed which provides increased thrust upon demand without increasing fuel flow unreasonably. In addition, a jet engine is needed which does not significantly increase drag.
Thus, it would be a distinct advantage to have a jet propulsion system and method which incorporates increased thrust upon demand without adversely increasing drag or utilizing unreasonably increased fuel flow. It is an object of the present invention to provide such a system and method.